In an inner-shell electrons state of a carbon on the surface of the electron beam source, an integrated intensity ratio of a sub-peak to a main peak is not less than 0.1 in measuring with X-ray photoelectron spectrometry. また、前記電子線源表面の炭素の内殻電子状態を、X線光電子分光で測定した場合の主ピークに対するサブピークの積算強度比が0.1以上とされた電子線源。 - 特許庁